1. Technical Field
The present invention relates to sulfide-based lithium-ion-conducting solid electrolyte glass, all-solid lithium secondary batteries, and methods for manufacturing all-solid lithium secondary batteries.
2. Related Art
Along with development of portable equipments such as personal computers and cellular phones, demand for small-sized lightweight secondary batteries as power sources of the portable equipments shows a drastic increase in recent years. Among such secondary batteries, in particular, lithium secondary batteries realize a high energy density since lithium has a reduced atomic weight and increased ionization energy. Extensive research has been made on such batteries, and as a result, the lithium secondary battery is presently used in a wide range of applications including a power source of the portable equipments. Such lithium secondary batteries may be roughly divided, according to the kind of electrolyte, into lithium ion batteries using liquid electrolyte, lithium ion polymer batteries using polymer solid electrolyte, and lithium ion batteries using inorganic lithium-ion-conducting solid electrolyte.
Among the lithium ion batteries described above, the all-solid lithium secondary battery typically has a structure shown in FIG. 1. Specifically, a positive electrode (I) is inserted in an insulating cylindrical tube 104 composed of polypropylene resin. The positive electrode (I) is formed by compression-molding positive electrode mixed material 103 composed of positive electrode active material and solid electrolyte powder in a metal mold under a pressure of about 4 tons/cm2. In this instance, a positive electrode current collector 102 electrically bonded to a positive electrode lead plate 101 is inserted in the positive electrode (I). Also, a negative electrode (II) is formed by compression-molding negative electrode mixed material 107 composed of negative electrode active material and electrolyte powder, with a negative electrode current collector 106 inserted in the negative electrode mixed material 107. A lithium-ion-conducting solid electrolyte layer 108 is placed between the positive electrode (I) and the negative electrode (II), and the entire structure is subject to a compression in a press machine, whereby the positive electrode layer, the electrolyte layer and the negative electrode layer are integrated, thereby forming an all-solid lithium secondary battery device.
The lithium-ion-conducting solid electrolyte layer 108 may be formed from sulfide-based lithium-ion-conducting solid electrolyte mixed with α-alumina (Al2O3), a material that can improve the ion conductivity of sulfide-based lithium-ion-conducting solid electrolyte that is a base material. It is known that all-solid lithium secondary batteries using such electrolyte layers exhibit improved charge-discharge cycle characteristics. The battery device is formed in a manner that the positive electrode (I) and the negative electrode (II) are filled in the insulating polypropylene resin cylindrical tube 104 under pressure, and strongly tightened with bolts and nuts through insulating tubes for preventing short-circuit between the positive electrode (I) and the negative electrode (II) by press forming metal molds which also serve as electrode terminals. It is noted that the manufacturing process described above is conducted in a dry inert gas atmosphere in a room temperature area.
Also, for the sulfide-based lithium-ion-conducting solid electrolyte 108 used in the battery device described above, sulfide-based lithium-ion-conducting solid electrolyte is mainly used, and crystal and amorphous electrolyte are used as the electrolyte. Battery devices manufactured with such materials are in a state in which the entire body of the battery device is pressurized, compressed and strongly consolidated within the insulating cylindrical tube 104. As a result, it becomes possible to avoid bonding failures at the bonding interfaces between the electrode active materials and the electrolyte powder which may be generated with volume expansion and contraction of the electrode active materials through battery's charge-discharge cycles. Accordingly, a reduction in the battery discharging capacity through charge-discharge cycles can be prevented, and the battery device exhibits excellent characteristics. If the battery device are not strongly confined in the insulating cylindrical tube 104, the battery discharging capacity would considerably reduce with its charge-discharge cycles.
As an example of another all-solid lithium secondary battery, S. D. Jones and J. R. Akridge, J. Power Sources, 43-44, 505 (1993) discloses an all-solid thin film lithium secondary battery produced by sequentially forming a negative electrode thin film, an electrolyte thin film and a positive electrode thin film through the use of a deposition apparatus or a sputtering apparatus. It is reported that the thin film lithium secondary battery exhibits superior charge-discharge cycle characteristics of several thousand cycles or more. The battery is made of a single thin plate of electrolyte formed by deposition without having grain boundaries within the electrolyte layer, such that migration of lithium ions is not influenced by bonding grain boundaries of the electrolyte particles, which makes it difficult for grain boundary junction failures to occur against volume expansion and contraction through charging and discharging operations of the electrode active materials, such that the thin film lithium secondary battery exhibits superior charge-discharge cycle characteristics.
However, in the all-solid lithium secondary battery described above, the solid electrolyte in the electrolyte layer and the electrode layers is formed from electrolyte powder particles being simply consolidated by compressive pressure, and therefore particles exist at their contact interfaces, and their bonding force is weak. In particular, when the all-solid lithium secondary battery is quickly charged, differences are generated in the current density distribution within the battery. Portions having strong current density distribution would cause a very large change in the electrode volume, which cause electronic bond failures at the bonding interface between the electrode active material and the electrolyte particles.
Also, in battery systems that use reversible deposition reaction of metal lithium as the reaction of negative electrode active material, lithium ions would deposit in lithium metal dendrites at bonding grain boundaries among the electrolyte powder particles within the electrolyte layer. As a result, this type of all-solid lithium secondary battery eventually has lowered charge-discharge current density and its capacity also gradually reduces along with charge-discharge cycles. Furthermore, the dendrite lithium metal expands bonding interfaces among electrolyte particles, and deposits in the bonding interfaces, which lead to various problems, such as, electrical short-circuit between the positive electrode and the negative electrode, and the like.
Furthermore, in order to make the all-solid thin film lithium secondary battery devices to be abundantly practical, they should have a higher capacity. In order to achieve this, the amount of electrode active material to be used needs to be increased while maintaining the ion-conducting paths within the electrode layers. If the electrode layer is to be made thicker by using the same technology, without changing the configuration of the electrode, the electrode resistance becomes greater. In order to lower the resistance, ion-conducting electrolyte material needs to be deposited among the electrode active material simultaneously when depositing the electrode. As a result, this process would not only increase the deposition time, but also require an expensive apparatus, such as, a multi-source deposition apparatus for deposition, such that the cost of manufacturing the all-solid thin film lithium secondary batteries becomes substantial. It is extremely difficult to increase the amount of electrode active material by the conventional deposition methods, and practical lithium secondary batteries with high energy cannot be provided at low costs.
In order to solve the problems described above, lithium-ion-conducting electrolyte layers to be used are provided with flexibility, thereby providing the layers with improved workability. In this respect, organic polymer binder is added to the electrolyte powder used in the structure of lithium-ion-conducting electrolyte layer, which is formed into a flexible sheet. A variety of such sheets have been studied, but in any of the solid electrolyte sheets formed, grain boundaries are present at bonding interfaces among electrolyte particles. When all-solid lithium secondary batteries are formed with such a solid electrolyte sheet, the phenomenon described above occurs in charge-discharge cycles, and in particular, deep charge-discharge cycles of the batteries manufactured, in which the current density distribution varies within the battery, and portions having strong current density distribution would cause a very large change in the electrode volume, which cause electronic junction failures at the bonding interfaces of particles between the electrode active material and the electrolyte.
Also, in battery systems that use reversible deposition reaction of lithium metal as the reaction of negative electrode active material, lithium ions would deposit in lithium metal dendrites at grain boundaries among the electrolyte powder particles within the electrolyte layer. As a result, this type of all-solid lithium secondary battery eventually has lowered charge-discharge current density along with charge-discharge cycles, and its capacity also gradually reduces along with charge-discharge cycles. Furthermore, the dendrite lithium metal expands bonding interfaces among electrolyte particles, and deposits in the bonding interfaces, which lead to various problems, such as, electrical short-circuit between the positive electrode and the negative electrode, and the like.
Also, in solid electrolyte sheets which are formed with addition of organic polymer binder, the conductivity of lithium ions tends to lower considerably, compared to solid electrolyte single body without any binder added. Therefore, as the lithium-ion-conducting solid electrolyte to be used, those with excellent ion conductivity need to be used. Therefore, for example, Li2S—SiS2, Li2S—SiS2—LiI, Li2S—SiS2—LiBr, Li2S—SiS2—LiCl, Li2S—SiS2—B2S3—LiI, Li2S—SiS2—P2S5—LiI, Li2S—B2S3, Li2S—B2S3—LiI, Li2S—P2S5, Li2S—P2S5—LiI, Li2S—P2S5-ZmSn (Z=Ge, Zn, Ga)(m is integer except 0)(n is integer except 0), Li2S—GeS2, Li2S—SiS2—Li3PO4— based sulfide lithium-ion-conducting solid electrolyte glass, crystalline lithium ion conductor including any one of the foregoing compositions, or lithium-ion-conducting solid electrolyte formed from a mixture of the foregoing compositions are used. Above all, multi-source based lithium ion conducting solid electrolyte materials other than Li2S—B2S3 and Li2S—P2S5 have been studied, because they exhibit excellent lithium ion conductivity. However, most of them include semiconductor material or halogenated lithium, such as, Si, Ge, LiI and the like as constituent materials.
Among sulfide-based lithium-ion-conducting solid electrolyte materials that exhibit excellent ion conductivity, many of those materials contain Si and Ge. When these materials are used as electrolyte in all-solid lithium secondary batteries, in its charge-discharge reactions, particularly at the negative electrode, Si and Ge are reduced near the potential at which reduction of lithium ions into metal lithium advances. Therefore, carbon that is generally used as the negative electrode active material of the lithium ion batteries cannot be used. Accordingly, as the negative electrode active material, indium (In) that is a material having a reversible reaction potential higher than that of lithium has been used. As a result, the operation voltage of the battery thus formed is lower than that of batteries using carbon as a negative electrode material. In other words, compared to batteries using carbon as a negative electrode material, all-solid lithium secondary batteries using In as a negative electrode material are higher in costs and lower in operation voltages.
Also, as sulfide-based lithium-ion-conducting solid electrolyte material, sulfide-based lithium-ion conductor containing lithium iodide may be used as electrolyte of all-solid lithium secondary batteries. In this case, oxidation-reduction reactions of iodine advance around 3.0 V in the charge-discharge reaction, particularly at the positive electrode; and for example, the reaction (at about 4.2 V) occurring when lithium cobalt oxide is used as the positive electrode active material is inhibited. For this reason, material that exhibits potentially high charge-discharge reactions cannot be used as the positive electrode active material, such that all-solid lithium secondary batteries with low operation voltages could only fabricated.